1. Field of the Invention
The invention relates to the chemistry of biologically active compounds. More particularly to specifically substituted porphyrin and chlorin derivatives that can be used as photosensitizers for a wide range of light irradiation treatments such as photodynamic therapy of cancer, infections and other diseases.
2. Invention Disclosure Statement
Photodynamic therapy (PDT) is one of the most promising new techniques now being explored for use in a variety of medical applications (Photodynamic therapy, basic principles and clinical applications. Eds. B. W. Henderson, Th. J. Dougherty, Marcel Dekker, 1992, New York), and particularly is a well-recognized treatment for the destruction of tumors (Photodynamic tumor therapy. 2nd and 3rd generation photosensitizers. Ed. J. G. Moser, Harwood Academic Publishers, 1998, Amsterdam). Photodynamic therapy uses light and a photosensitizer (a dye) to achieve its desired medical effect. A large number of naturally occurring and synthetic dyes have been evaluated as potential photosensitizers for photodynamic therapy. Perhaps the most widely studied class of photosensitizers are the tetrapyrrolic macrocyclic compounds. Among them, especially porphyrins and chlorins have been tested for their PDT efficacy. Porphyrins are macrocyclic compounds with bridges of one carbon atom joining pyrroles to form a characteristic tetrapyrrole ring structure. There are many different classes of porphyrin derivatives including those containing dihydro-pyrrole units. Chlorins, as referred to in the present invention, are porphyrin derivatives containing one dihydro-unit whereas bacteriochlorins are characterized by two dihydro-pyrrole units (in general in chlorins one double bond of the aromatic system in β-position is absent and in bacteriochlorins two opposite double bonds are absent compared to the porphyrin). As examples of tetrapyrrolic macrocyclic compounds used as photosensitizers, U.S. Pat. No. 4,656,186 from Bommer et. al. discloses fluorescent mono, di- or polyamide of an aminocarboxilic acid and tetrapyrrole containing at least three carboxy groups, U.S. Pat. No. 7,022,843B1 from MacAlpine et. al. provides β,β′-dihydroxy meso-substituted chlorin as photosensitizers, and U.S. Pat. No. 7,166,719B2 from Pandey et. al. discloses tetrapyrrole compounds containing a fluorinated substituent where the compound is a chlorin or a bacteriochlorin for PDT diagnostic and therapeutic application.
There are several properties that an effective photosensitizer should accomplish. Among them, a desirable characteristic in order to efficiently destroy deep target tissues is a strong absorption at long wavelength. Many current photosensitizers are not efficient enough as they have low absorption in the red region of the spectrum. Chlorins have the advantage that they possess an intense absorption in the red and near-infrared region of the electromagnetic spectrum. As light of longer wavelength penetrates deeper into the tissue, it is thus possible to treat e.g. more expanded tumors, if the PDT is employed for tumor therapy. Chlorins possessing potential for PDT can either be derived from natural sources or from total synthesis.
If the chlorins are derived from natural compounds they are usually obtained by derivatizing chlorophylls or bacteriochlorophylls, as for example the photosensitizers derived from chlorophyll a of photosynthetic plants and algae disclosed in U.S. Pat. No. 5,330,741. Due to the sensibility of the natural compounds this is often difficult and requires vast resources. So, the synthesis of chlorins by total synthesis is an appealing alternative. Methods to prepare chlorins and bacteriochlorins by total synthesis are known in the art. Generally these compounds are prepared by first synthesizing the porphyrin and then converting the porphyrin system to a chlorin or bacteriochlorin system. This step can e.g. be performed by the reduction with in situ generated di-imine or by cis-dihydroxylation with osmium tetroxide; multistep reactions leading to trans-dihydroxylation are also known (patent EP 00337601B1; patent application WO 09613504A1, patent application WO 00061584A1; C. Brückner, D. Dolphin, 2,3-vic-Dihydroxy-meso-tetraphenylchlorins from the Osmium Tetroxide Oxidation of meso-Tetraphenylporphyrin, Tetrahedron Lett. 1995, 36, 3295-3298; C. Brückner, D. Dolphin, β,β′-Dihydroxylation of meso-Tetraphenylchlorins, Tetrahedron Lett. 1995, 36, 9425-9428; H. W. Daniell, S. C. Williams, H. A. Jenkins, C. Brückner, Oxidation of meso-tetra-phenyl-2,3-dihydroxychlorin: simplified synthesis of β,β′-dioxochlorins, Tetrahedron Lett. 2003, 44, 4045-4049; F. Rancan, A. Wiehe, M. Nöbel, M. O. Senge, S. Al Omari, F. Böhm, M. John, B. Röder, influence of substitutions on asymmetric dihydroxychlorins with regard to intracellular uptake, sub cellular localization and photosensitization in Jurkat cells, J. Photochem. Photobiol. B: Biology 2005, 78, 17-28; I. Laville, T. Figueiredo, B. Loock, S. Pigaglio, Ph. Maillard, D. S. Grierson, D. Carrez, A. Croisy, J. Blais, Synthesis, Cellular Internalization and Photodynamic Activity of Glucoconjugated Derivatives of Tri and Tetra(meta-hydroxyphenyl)chlorines, Bioorg. Med. Chem. 2003, 11, 1643-1652). Mostly, compounds with four identical substituents in the meso-positions have been investigated and tested for their PDT efficacy. One prominent example is Temoporfin which is the active compound in the medicinal product Foscan® which is successfully used in Europe as a medicinal product for the PDT treatment of head and neck cancer. Also, all examples in the abovementioned patent application WO 09613504A1 are compounds with four identical meso substituents. The few publications on unsymmetrically tetrakis-meso-substituted chlorins derived from total synthesis that exist are of the so-called A3B-type, i.e. incorporating 3 identical and one different meso-substituent (I. Laville, T. Figueiredo, B. Loock, S. Pigaglio, Ph. Maillard, D. S. Grierson, D. Carrez, A. Croisy, J. Blais, Synthesis, Cellular Internalization and Photodynamic Activity of Glucoconjugated Derivatives of Tri and Tetra(meta-hydroxyphenyl)chlorines, Bioorg. Med. Chem. 2003, 11, 1643-1652, F. Rancan, A. Wiehe, M. Nöbel, M. O. Senge, S. Al Omani, F. Böhm, M. John, B. Röder, Influence of substitutions on asymmetric dihydroxychlorins with regard to intracellular uptake, sub cellular localization and photosensitization in Jurkat cells. J. Photochem. Photobiol. B: Biology 2005, 78, 17-28; J. K. Macalpine, R. Boch, D. Dolphin, Evaluation of tetraphenyl-2,3-dihydroxychlorins as potential photosensitizers, J. Porphyrins Phthalocyanines 2002, 6, 146-155). One reason for using symmetrically substituted porphyrins to convert them into chlorins is that in this case no isomers are formed. If no isomers are formed the resulting compounds are easily characterized and prepared, a key factor for commercial production. If unsymmetrically substituted porphyrins are used to convert them into chlorins different regioisomers are formed which require subsequent separation (not in the case of a trans-arrangement of the substituents, cf. FIG. 2). Therefore, the chlorins with a meso-A3B-substitution found in the art are often poorly characterized or are used as an isomeric mixture without separation (e.g. J. K. Macalpine, R. Bach, D. Dolphin, Evaluation of tetraphenyl-2,3-dihydroxychlorins as potential photosensitizers, J. Porphyrins Phthalocyanines 2002, 6, 146-155; I. Laville, T. Figueiredo, B. Loock, S. Pigaglio, Ph. Maillard, D. S. Grierson, D. Carrez, A. Croisy, J. Blais, Synthesis, Cellular Internalization and Photodynamic Activity of Glucoconjugated Derivatives of Tri and Tetra(meta-hydroxyphenyl)chlorines, Bioorg. Med. Chem. 2003, 11, 1643-1652). As it is difficult to purify the mixture in order to eliminate the isomers that do not contribute to the PDT effect or enrich the preparation with active compounds, it would be an advantage to find alternative unsymmetrically tetrakis-meso-substituted chlorins easily characterized and produced with simple preparation methods. Especially to seize the particular properties of unsymmetrically substituted chlorins, as they might increase the amphiphilicity of the compounds and thus their membrane affinity and PDT efficacy.
Thus, there is a need to enhance the effectiveness of prior art biologically active compounds used as photosensitizers in order to successfully perform a wide range of light irradiation treatments such as photodynamic therapy of cancer, infections and other diseases. Moreover, it is necessary to provide novel methods of preparation and application of unsymmetrically tetrakis-meso-substituted chlorins in order to provide enhanced photosensitizers than those available up to date. Thus, PDT efficacy would be increased by taking advantage of unsymmetrically tetrakis-meso-substituted chlorins properties, such as strong absorption at long wavelength of the red and near-infrared region of the electromagnetic spectrum for deeper tissue penetration, enhanced selectivity for tumors or other target tissues over healthy surrounding tissues due to its tailored amphiphilicity that increases membrane affinity, and custom-made pharmacokinetic behavior depending on the particular PDT application.